The use of phase-locked loops (PLL) with arbitrary frequency division (by rational division ratios N) is a well-known method for synthesizing desired frequencies. So-called fractional-N indirect digital frequency synthesis, i.e., using a PLL, is particularly well suited to integrated circuit applications. The technique allows very narrow channel spacing relative to the output frequency, large bandwidth in the PLL relative to the channel spacing, and high output frequency relative to the processing technology used in the integrated circuit. Consequently, the availability of a low-noise, low-spurious-frequency form of fractional-N division can have a significant impact on the performance of low-cost frequency synthesizers for use in consumer products.
By using fractional-N division, the wider loop bandwidth for a given channel spacing allows faster settling time and reduced phase noise requirements to be imposed on the voltage-controlled oscillator (VCO). With reduced phase noise requirements, lower cost, possibly on chip, VCO's might be used. The faster settling time, resulting from broader loop bandwidth of a PLL-based fractional-N frequency synthesis, has the potential to eliminate additional hardware typically included in a PLL to provide fast settling. Thus, a high-performance fractional-N frequency synthesizer technique may lead to a complete high-performance synthesizer on a chip.
It is known to use a Σ-Δ modulator to instantaneously alter the feedback division modulus (see, e.g. Tom A. D. Riley: Delta-Sigma Modulation in Fractional-N Frequency Synthesis, in: IEEE Journal of Solid-State Circuits, vol. 28, no. 5, May 1993). However, this typically introduces excessive phase noise and fractional spurs. The use of third order multi-stage noise-shaping modulators (3rd order MASH modulators) are usually able to provide high stability and sufficient noise shaping characteristics. However, particularly when the rational division ratio is close to an integer number the significant spurs (called “fractional spurs”) may occur in the spectrum of the PLL output signal. Thus there is still a general need for an improved frequency division with rational division ratios (a so-called fractional-N division) providing an improved rejection of fractional spurs.